1. Field of the Invention
The present invention relates generally to the support of marine risers, such as offshore well production risers of deepwater spar type drilling and production platforms, which risers extend upwardly from the seabed to a drilling or production deck or working floor. More particularly, the present invention is directed to a system for combining the, upward forces of buoyant members to the riser or risers of deepwater spars and other marine platforms to thus assist in supporting the weight of the risers. The present invention also concerns selective installation of inflatable buoyant members to the risers during or after riser tieback to provide buoyant riser weight offsetting force along the length of the riser or at or near the upper stem joint of the riser. This invention further concerns the use of buoyancy modules which collapse to a small dimension for installation or retrieval through rotary drilling table openings or small openings in the deck structure of the spar and can be inflated with a gas or an uncured liquid foam composition and/or provided with liquid ballast individually, sequentially or simultaneously as desired.
2. Description of the Prior Art
On deepwater spars, metal buoyancy tanks, also referred to as “cans”, are used to support the weight of production risers within the spar. Currently these buoyancy tanks are installed by two methods. In some cases buoyancy tanks are pre-installed into the spar structure prior to its launching. Alternatively, the buoyancy tanks may be installed after the spar is launched, by using one or more heavy lift vessels or derrick barges. The requirement for buoyancy installation at remote marine sites and the use of heavy lift vessels or derrick barges for buoyancy installation obviously adds significantly to the cost and complexity of buoyancy tank installations. The large dimensions and heavy handling weight of typical buoyancy cans, and the minimal size of most spar deck openings makes it ordinarily impossible to attach buoyancy cans to the riser structure at the level of the deck and then lower the buoyancy cans to the desired water depth thereof along with the riser during its installation and tieback. Thus, specialized and expensive buoyancy can installation equipment, typically in the form of an installation barge, is ordinarily required. The buoyancy tanks or cans are typically connected to various joints of the riser assembly so that buoyancy force is applied to the riser at selected locations along its length.